Add support for playing an AudioBufferSourceNode at a negative playbackRate. Change the meaning of
start() to set the initial playback position at the end of the play range if the rate of playback
is negtive.

* Modules/webaudio/AudioBufferSourceNode.cpp:
(WebCore::AudioBufferSourceNode::AudioBufferSourceNode): Allow the playbackRate AudioParam to range from [-32, 32].
(WebCore::AudioBufferSourceNode::renderFromBuffer): Change variable names from "start" and "end" to "min" and "max"
for clarity. Add a non-interpolated and interpolated render step for negative playback.
(WebCore::AudioBufferSourceNode::start): Drive-by fix: default value of grainDuration is not 0.02.
(WebCore::AudioBufferSourceNode::startPlaying): Start playing at the end of the buffer for negative playback.
(WebCore::AudioBufferSourceNode::totalPitchRate): Allow the pitch to be negative.